Chế tạo và nghiên cứu tính chất chuyển trong vật liệu Perovskite đơn và kép có nhiệt độ chuyển pha Curie cao : Đề tài NCKH. QG.06.05

Ngoai hieu ung CMR hua hen nhi^u kha nang ling diing het siic to Ian nguai ta con nhan thay rang cac hop cha't perovskite bieu hi6n nhieu tinh chat vat ly het sue ly thii va phiic tap kh

DAI HOC QUOC GIA HA NOI TRUCING DAI HOC KHOA HOC TU NHIEN

TEN DE TAI

CHE TAO VA NGHlfeN CtfU TINH CHAT CHUYEN TRONG VAT

LIEU PEROVSKITE DON VA KEP CO NHIET BO CHUYEN PHA

CURIE GAG

MA SO: QG - 06 - 05

V >K- V CHU TRI DE TAI

GS TS Nguyen Huy Sinh

DAI HOC QUOC GIA HA NOI TRUdNG DAI HOC KHOA HOC TU NHIEN

TEN f)i TAI

CHE TAO VA NGHIEN CtTU TINH CHAT CHUYEN TRONG VAT

LIEU PEROVSKITE DON VA KEP CO NHIET BO CHUYEN PHA

CURIE CAO

MA SO: QG - 06 - 05

CHU TRI D £ TAI: GS TS Nguyen Huy Sinh

CAC CAN B O T H A M GIA:

1 Pham Hong Quang PGS TS

2 VuTheHien ThS

3 TranDinhTho NCV

4 Vu Thanh Mai NCS

5 VQVanKhai ThS

6 Hoang Diic Quang NCS

7 Nguyen Anh Tuan NCS

8 Tran Minh Due Hoc vien Cao hoc

9 D 6 Viet Thang Hoc vien Cao hoc

HA NOT - 12 - 2007

A BAG CAO TOM T A T

a) Ten de tai: Che tao va nghien cihi cac tinh chat chuyen trong vat lieu

Perovskite don va kep c6 nhiet dp chuyen pha Curie cao

Ma so: QG - 06 - 05

b) Chii tri de tai: GS TS Nguyen Huy Sinh

Chii nhiem Bo mon Vat ly Nhiet do thap

Khoa Vat ly, Truomg Dai hoc Khoa hoc Tir nhien, DHQG Ha noi

c) Cac Can bg tham gia

1 Pham Hong Quang PGS TS

2 VuTheHien ThS

3 TranDinhThg NCV

4 Vu Thanh Mai NCS

5 VuVanKhai ThS

6 Hoang Du-c Quang NCS

7 Nguyen Anh Tuan NCS

8 Tran Minh Due Hgc vien Cao hoc

9 D6 Viet Thing Hoc vien Cao hgc

d) Muc tieu va ngi dung nghien ciiu

L Muc tieu ciia de tai:

Che tao cac mSu nghien ctru c6 cau true perovskite dan pha va c6 nhiet do chuyen

pha Curie a gan nhiet do phong

Nghien cim tinh chat chuyen trong cac mSu che tao Cu the nhu cac tinh chat: nhiet, dien, tir, su chuyen cau true va mot vai hieu iJng chuye'n pha khac

Dao tao cac Cu nhan Dai hgc, Thac si va Tien sL

2 Ngi dung nghien cufu cua de tai:

• Tim kiem qui trinh cong nghe on dinh che tao cac he mSu perovskite loai ABO3 c6

cau true dan pha

• Nghien ciiu cac tinh chat chuyen nhu nhiet, dien, tir va cac tinh chat khac cua cac

he mau Rei.,Ca,(Mn|.,Ay)03 vai Re la cac nguyen to dat hiem va A la cac nguyen

16 kim loai chuyen tiep 3d

• Nghien ciru mot vai linh chat cua Van-spin va hieu tirgng ban din xuyen ngSm

e) Cac ket qua dat diigc

1 Nghien cufu

+ Tim kiem va on dinh qui trinh c6ng nghe che tao cac mSu gom Perosvkite

+ Che tao cac he mSu Perovskite Rei.xCa;^(Mni.yAy)03 c6 cau true dan pha va c6 nhiet

do chuyen pha Curie gan nhiet d6 phong

+ Cong bo cac kel qua nghien curu ve cac tinh chat chuyen cua mot so he vat lieu perovskite Re,_xCax(Mni.yAy)03 tren mot vai tap chi Khoa hgc va cac Hoi nghi Khoa hgc trong nude va Quoc te

2 So cong trinh Khoa hoc da cong bo: 07

+ So cong trinh Khoa hgc cong bo tren cac tap chi Quoc te: 01

+ So cong trinh Khoa hgc cong bo tren cac tap chi Quoc gia: 02

+ So cong trinh Khoa hgc cong bo tren tuyen tap HNKH Quoc te: 02

+ So cong trinh Khoa hgc cong bo tren tuyen tap HNKH Quoc gia: 02

[1] Nguy6n Huy Sinh, Vu Thanh Mai, D6 Viet Thdng Tinh chat vat ly cua hap chat

thieu La()45CaQ^^3Mn03.s- Journal of Science and Technology Tap 44-s6-3-(2006)

Exchange bias and Related Effects in Spin Valves Proceedings of the 1th

IWOFM 3 IWONN conference, Ha Long, Viet Nam December 6-9, 2006,

788-791

[4] Nguyen Huy Sinh, Hoang Due Quang, S C Yu, M Mayer

Suppression of Magneto-Current in FeCo-Base Magnetic Tunneling Transistors

VNIJ, Journal of Science TXXH, No 2AP (2006) 135-138

[5] Nguyen Huy Sinh, Vu Van Khai, Nguyen Thi Thuang, Ti^ tra trong cac hop chat La2/3Ca,;,Mn,.JM,03 (TM = Cu, Zn), Tuyen tap hoi nghi Vat ly chat ran toan quoc Ian thu 5, Vung Tau 11/2007

[6] H D Quang, N H Sinh, S K Oh, P T Son, N Q Hoa, V V Khai, S C Yu,

Conductance Behaviors of Magnetic Tunneling Transistors with FeCo Base

Proceedings of the ChinaNAN()2007_International Conference on Nanoscience & Technology, June - 2007 Beijing, China

[7] Vu Van Khai, Nguyen Thi Thuang, Nguyen Huy Sinh, D6 Viet ThAng

Hieu ting thay theCoban nong do thdp trong cac hap chat La2i3Pbj,3MnOj Tuyen

tap hoi nghi \M ly chat r^n toan quoc Ian thu 5, VQng Tau 11/2007

3 So seminar da trinh bay trong qua trinh thuc hien de tai: 05

1) De tai: Mot so tinh chat giong va khac nhau trong Perovskite dan va kep

Bao cao vien: PGS TS Nguyen Huy Sinh

C h u t r i d 6 t a i : Q G - 0 6 - 0 5

Ngay 20/09/2006 tai bg m6n vat ly nhiet do thap

2) De tai: Thay the kim loai chuyen tiep Fe, Co, Ni cho Mn trong hgp chat La2/3Cai/3Mn03

Bao cao vien: Nguyen TuS'n San

Chu trl de tai PGS TS Nguyen Huy Sinh

Ngay 15/11/2006 tai Bg mon Vat ly Nhiet do thap

3) De tai: Chuyen pha tix va mot so hieu ling thay the trong cac Perovskite

Manganite

Bao cao vien: Vu Thanh Mai (ngi dung chinh luan an Tien sT)

Chu tri de tai: PGS TS Nguyen Huy Sinh

Ngay 30/3/2007 tai Bg mon Vat ly Nhiet do thap

4) De tai: Tir tra khong 16 trong cac hgp chat Perovskite Manganite

Bao cao vien: PGS TS Nguyen Huy Sinh

Ngay 29/06/2007 tai Bg mon Vat ly Nhiet do thap

5) De tai: Nghien ciiu tinh chat dien va tir trong he hgp chat Perovskite La2/3Cai/3Mn03.5pha tap Zn

Bao cao vien: Vu Van Khai (ngi dung luan van Thac si)

Chii tri de tai PGS TS Nguyen Huy Sinh

Ngay 17/12/2007 tai Bg mon Vat ly Nhiet do thap

c) Huang d^n luan van Thac sT: 03 luan van Thac si va 01 khoa luan Thac si

1) Nguyen Tuan San Bao ve thang 11/2006

De tai: Anh huang ciia sir thay the kim loai tij tinh 3d cho Mn trong cac hgp chat

2) Vu Van Khai Bao vft thing 12/2007

D6 tai: Nghiftn clhi tinh cha't dien va tir trong he hop chat perovskite

^^2/3Cai;3Mn03^ pha tap Zn

Can bo hu6ng dSn luSn vSn: PGS TS Nguyen Huy Sinh

3) Tran Minh Du-c

E>^ tai: M6t s6' tinh ch^t chuyen trong he hgfp cha't Perovskite La,.,Pb,Mn03

Hoc vien da hoan thinh luin van nhung xin hoan d^ di cong tac a nude ngoai

4) Pham Thi Thuang

De tai: Spintronics (2006)

d) Hudng din luftn an Ti^n si: 01 luan an tien si va 03 chuyen de Tien sT

Vu Thanh Mai Bao vfi thing 5/2007

De tai: Nghifin curu cac chuyen pha va hieu irng thay the trong cac perovskite manganite

Can bo hudng dSn: PGS TS Nguyen Huy Sinh

f) Tinh hlnh kinh phi cua de tai:

Kinh phi duoc cS'p nSm 2006: 35.000.000 (ba muoi lam trieu dong chan)

Kinh phi duoc cap nam 2007: 25.000.000 (hai muoi lam trieu dOng chan)

Da chi theo dir toan trong cac hop dong nghien ciiu khoa hoc hang nam

KHOA QUAN L t CHU NHIEM DE TAI

PGS TS Nguyen The Binh GS TS Nguyin Huy Sinh

XAC NHAN CUA CO QUAN QUAN LY

• H 6 MI^U TRUONG

GS.JS ^uluJAi^i^iJum,

A SHORT REPORT OF THE SCIENCE PROJECT:

Title: Preparation and study of the transport properties in the single and double - perovskite materials with high Curie - Temperature

Code: QG - 06 - 05

) Coordinator of Project:

Prof Dr Nguyen Huy Sinh

Head of the Low - Temperature Physics department

c) Participants of the Project:

Vu Thanh Mai

Vu Van Khai Hoang Due Quang Nguyen Anh Tuan Tran Minh Due

Do Viet Thang

1 and content of study in project

1 Aims of the project are in the following:

+ Preparation of the samples with perovskite structure single phase and their Curie temperatures near the room temperature

H- Study of the transport properties of the prepared samples, such as thermodynamic, electric and magnetic properties, change structure and some other phase transitions + Training for the Bachelor of Physics, Masters and Doctors

+ Report of the study results in Scientific Journals and on the workshop or Conference

2 Contents of the project:

Find a stable Technology process to prepare the series of the perovskite samples of ABO:i type with single phase structure

+ Study of the transport properties of the series of Lai.,Ca,(Mn,.yA,)0:( provskite samples with Re are Rear - Earth elements and A a 3d - transition metals

+ Study of a some properties on the Spin - Valve (collaboration between our study group and Applied Physics laboratory, Department of Physics Chungbuk National

e) The obtained results

1- For studying:

+ It is found the convenient Technology process to prepare the Perovskite samples

with single phase structure and their Curie temperatures near the room temperature region

+ Successfully prepared the perovskite samples of Re,.^Ca,(Mni.yAy)0^ systems with high-quality for study

+ Measure the physical parameters describing the transport properties of the prepared samples above, and reported the obtained results on the Scientific Journal and on the proceeding of the workshop or Conference inside or outside of Vietnam

+ Numbers of the Publication: 07 Which are including:

• Publication in the international Science Journals: 01

• Publication in the national Science Journals: 02

• Publication in the proceeding of Scientific international Conference: 02

• Publication in the proceeding of Scientific National Conference: 02

[1] Nguyen Huy Sinh, Vu Thanh Mai, D5 Viet Thang

Physical properties of Lantan deficient Lai, f^Caf^j^MnO3,s compounds Journal of

Science and Technology Vol 44; N^ 3-(2006) 123-130

[2] Hoang Due Quang, Nguyen Huy Sinh, Suhk-Kun OH, Thanh Nhan Huynh, Nguyen The Hien, Jerry ZiDanic and Seong Cho - Yu

Reaction Mechanism of Chemical element (Co, Fe, Mn) existing in spin valves

containing Oxide Layers Japanese Journal of Applied Physics Vol 45 N" lA,

(2006) 88-92

[3] Hoang Due Quang, Cao Xuan Huu, Nguyen Huy Sinh, Suhk Kum Oh, Nguyen The Hien, Vu Van Khai and Seong-Cho-Yu

Exchange base and Related Effects in Spin Valves Proceedings of the 1th

IWOFM 3 IWONN conference, Ha Long, Viet Nam December 6-9, 2006,

788-79L

[4] Nguyen Huy Sinh, Hoang Due Quang, S C Yu, M Mayer,

Suppression of Magneto-Current in FeCo-Base Magnetic Tunneling Transistors

VNIJ, Journal of Science T.XXE, No 2AP (2006) 135-138

[5] Nguyen Huy Sinh, Vu Van Khai, Nguyen Thi Thuong

Magnetoresistance in La2j3Cajf3Mn,JM,03 compounds (TM = Cu, Zn),

Proceedings of the fifth National Solid State physics Conference, Vung Tau

11/2007

[6] H D Quang, N H Sinh, S K Oh, R T Son, N Q Hoa V V Khai S C Yu

Conductance Behaviors of Magnetic Tunneling Transistors with FeCo Base

Proceedings of the ChinaNANO 2007 International Conference on Nanoscience & Technology, June - 2007 Beijing, China

[7] Vu Van Khai, Nguyen Thi Thuong, Nguyen Huy Sinh, Do Viet Thang

Effect of low concentration of Coban substitution in La2i3Pbi,3Mn03 compounds

Proceedings of the fifth National Solid State physics Conference, Vung Tau

11/2007

+ Number of the Scientific Seminars have been presented during completing

project: 05:

L Assoc Prof., Dr Nguyen Huy Sinh

Title: Comparison between single and double perovskite compounds

Code: Q G - 0 6 - 0 5

Date: 20 September 2006

Place: Low Temperature Physics Department

2 MSc Nguyen Tuan Son

Title: Substitution of 3d - transition metals for Mn in La2/iCai/3MnO:i compounds Code: Q G - 0 6 - 0 5

Date: 15/11/2006 Place: Low Temperature Physics Department

3 PhD Student: Vu Thanh Mai

Supervisor: Assoc Prof., Dr Nguyen Huy Sinh

Title: The magnetic phase trasition and some substituted effect in perovskite Manganites

Code: QG - 06 - 05

Date: 30 March 2007 Place: Low Temperature Physics Department

4 Assoc Prof., Dr Nguyen Huy Sinh

Title: Giant magnetoresistance in manganite perovskite compounds

Code: QG - 06 - 05

Date: 17 Deceniber 2007 Place: Low Temperature Physics Department

5 MSc Vu Van Khai

Supervisor: Assoc Prof., Dr Nguyen Huy Sinh

Title: Study of the electronic and magnetic properties in La2/3Ca,/3Mni.,Zn,Ov6 perovskite system

Code: Q G - 0 6 - 0 5

Date: 30 March 2007 Place: Low Temperature Physics Department

2 For training:

- Coordinators of 4 students for practising to study: Do Viet Thang, Nguyen Thi

Nhung, Tran Thi Ha and Tran Qui Long

+ 04 Bachelors: Tran Thi Ha, Nguyen Thi Nhung, Tran Tuan Khoi and Nguyen Thi

Thuong

+ 03 Masters of Science: Nguyen Tuan Son, Vu Van Khai, Tran Minh Due

+ 01 Doctor of Physics: Vu Thanh Mai

B. PHXN CHiNH BAO CAO

Bao cao torn t i t

Short report of the science project

M a d a u

Noi dung chinh

Nghien cuti khoa hoc

Dao tao

Ket luan

Tai lieu tham khao

Bien ban kiem tra tien do thiic hien de tai Dae biet nam 2006

Phu luc

M^u 1: Tom tdt cac c6ng trinh nghien cuu khoa hoc cua ca nhan

M.1u 2: Scientific Project

Phieu ddng ky ket qua nghien curu

Vat lieu CO c&w true perovskite ABO3 (hinh 1), trong do A la cac nguyen to dat

hiem va B la cac 1dm loai chuyen tififp dirgfc phat hien trong the ky 18 No da dugc nghien cuu nhieu trong nhiing nSm 50 va dSc biet quan tarn a nhirng nam cuoi the ky 20 He vat Heu tieu bieu dugc tap trung nghiSn ciJu nhieu nhat do la La,.^Ca^MnOv6- Dae biet la tir sau khi phat hien ra hidn tudng siSu din nhiet dp cao, "Tir tra khong 16" (Colossal Magnetoresistance - CMR) trong h6 va cac nha khoa hoc cho rang hieu ung nay hiia hen

ma ra mot kha naiig irng dung v6 cilng to Idfn vi sii thay ddi dien tra ciia vat lieu c6 the dat

tai hang trieu l^n khi dat trong tu tnrdng cd khoang 10 T Day la mot sir thay doi khOng 16

cua dien tra ma chua tutig duoc quan sat tha'y trong bat ky vat lieu nao truac do Ngoai hieu ung CMR hua hen nhi^u kha nang ling diing het siic to Ian nguai ta con nhan thay rang cac hop cha't perovskite bieu hi6n nhieu tinh chat vat ly het sue ly thii va phiic tap

khcic da dugc m6 ta trong gikn do pha n6i tie'ng ciia Schiffer va cac cong sir [hinh 2] Gian d6 nay da cho biet nhitng tinh chfft diin hinh cua he La,.^Ca^Mn03.6 khi x bien d6i tii" 0 dcMi 1 Su thay the tang din c^c ion Ca^"*" vao vi tri La"*"^ da lam thay doi tral tu ciia he, lam

meo cau triic, dan de'n cac chuySn pha nhu sat tiif (FM) thuan tii (PM); sat tir (FM) phan sat tii" (AFM); kim loai (MT) - dien moi (IS)/ban dan (SC), trat tu dien tich (CO) v

-10

Cac loai tuong tac giila ciic ion ctlng hoa tri - tuang tac sieu trao doi (SE) va cac ion khac hoa tri - tuong tac trao d6i k6p (DE) cung thay d6i va dan den su canh tranh tuang

tac DE - SE khi thay d6i x keo theo su thay d6i tinh chat ciia tiimg hgp chat Ca che va

ngu6n goc cua nhihig thay d6i do vSn con nhi^u y kien chua thong nhal De hieu va lam

sang to ban vin de nay, nhihig nam gin day nhieu nghien curu tren cac vat lieu perovskite

thuang tap trung chi y£fu v^o cic hop ch^t nin Manganite va Cobanlite Ve mat dinh tinh, tinh chat tir va d^n cua cdc hop ch^t Lai.jtCajjMn03 va Lai.,Ca^Co03 c6 nhieu dac diem

kha giong nhau Tuy nhien, vSa c6 nhihig dac diim khac nhau ca ban giiia hai he hgp chat

nay do c6 sir khac nhau cua m6o mang Jahn-Teller va do Co t6n tai nhieu trang thai spin khac nhau trong vat liSu C^u tnic co ban ciia he vat lieu Lai.^Ca;,Mn03.6 la bo khung bat dien MnOfi Viec thay th^ cac ion Ca^* cho La"*"^ chi lam tac dong den vung xung quanh

bat dien gay nen cac thay d6i vi tinh ch^t vat lieu Gan day, cac nha khoa hoc da tien hanh nghien ciiu thay th^ cdc nguySn t6 kim loai chuyen tiep true tiep vao vi tri Mn, tire la tac

dong true tiep vao bd khung ca'u true MnOs- C^c ket qua thu dugc cho thay thay the vao vi tri Mn bang cac nguyen t6 kim loai chuyin tie'p 3d lam giam kha manh nhiet dp chuyen pha Tc cung nhu d6 din, nhung lai 1km tang manh gia tri tir tra Mot so tac gia da giai thich la do anh huang ciia su pha loang phan mang tu' va sir suy giam cua cuang dp tuang tac trao d6i kep Tuy nhien ban ch^t vat 1^ va ngu6n goc ciia cac thay doi ve tinh chat trong cac hgp chat pha tap van con nhi6u van de can dugc nghien cuu chi tiet, ly giai, minh chiing ro rang va chi ti£ft ban

Phan 16n cac giai thich ly thuy^t cho cac tinh chat dien va tir ciia cac hgp chat perovskite deu dua vao CO ch6' tuo'ng tac trao doi kep (DE) Tuy nhien, cac ket qua nghien cuu gan day cho thay m6 hlnh ca chfi'tuong tac trao doi kep khong du giai thich dugc toan canh biic tranh vat ly trong c^c vat lifiu perovskite Viec bo sung vao mo hlnh nay cac hieu ling bien dang mang Jahn - Teller, cung nhu cac thay doi dong hpc spin trong viing nhiet

dp xung quanh Tc dong vai tro quan trong Ngoai ra hien tugng bat d6ng nhat va sir tach pha trong vat lieu cung c6 vai tro quan trong trong tinh dan ciia vat lieu Nguai ta con quan sat thay cau true te' vi anh hudng manh den hieu ling tir dien tra trong cac hgp chat perovskite manganite

Nhu vay ca ve mat 1^ thuyet vh thuc nghiem, vat lieu perovskite manganite deu la

doi tugng nghien cthi rSft hap din Su pha tap 16 trong gian tiep vao vi tri dat hiem (A) hay

true tiep vao vi tri Mn (B) d6u gay n6n nhihig thay doi ca ban ve tinh chat vat ly cua tirng hgp chat Nhiing thay d6i nky da c6 s6c 16i cuon cac nha khoa hpc ca trong linh vuc

nghien cuu co ban va ling dung V^ nghiSn cuu co ban can phai tlm hieu ngu6n goc va ca

che ciia nhirng thay d6i tinh chSl vat lieu V^ mat nghien curu ling dung, dieu quan trpng

nhat la phai nang cao dugc nhiet d6 chuyen pha Curie (Tc) ciia cac hgp chat perovskite manganite len vilng nhiet d6 phong vk nang cao ty so tit dien tro cua chiing nhat la trong nhung viing tir truing thSip Mat khdc, hieu utig tit nhiet Idfn ling dung trong cac thiet bi lam lanh the' he m6i cung da dugc nghien cuu manh va dat niem hy vpng vao vat lieu perovskite chiia mangan

Hgp chat LaMn03 thi hien tinh phin sat tir do tuang tac SE giiia cac ion Mn^^ Khi

pha tap kim loai ki6m th6 Ca (hoa tri 2) who vi tri dat hiem, de can bang hoa tri thi mot

phan tuang duong ion Mn^"*" chuyen thknh Mn"*"" Khi do trong hgp chat xuat hien ca tuang tac SE giiia cac ion cung hoi tri (Mn"*^- Mn"*^; Mn"^"*- Mn"*"^) va tuong tac DE giira cac ion

khac hoa tri (Mn'^\ Mn"^) Nhu vay ck hai loai tuong lac DE va SE ciing t6n tai va canh

tranh nhau trong hgp chSit pha tap Lai.xCaxMn03 Ca'u triic tir va tinh din dien ciia vat lieu perovskite La,.xCaxMn03 duoc quy^t dinh bofi cuang dp va tuong quan giiia hai loai tuang tac nay Cuang dp va tuong quan giiia hai loai tuong tac nay lai phu thupc vao nong dp thay the Ca cho La trong hop cha't V6i m6t qua trinh nghien cuu ti mi va cong phu bang ihuc nghiem Schiffer ciing cdc d6ng nghiep da xay dung dugc gian d6 pha ciia he Laj .Ca.MnO, nhu tren hlnh 2

Khi chua co sU pha tap (x = 0) thi hgp chat co tinh phan sat tir dien moi vdi cau iriic phcin sat tii' kieu A

Khi CO su pha tap vdi x < 0,1 thi tuong tac giQa cac ion d6ng hoa tri Mn^^- Mn^\ Mn^'^- Mn"^"^ chie'm uu the", he mang tinh phan sat tii' vai cau triic CAF

Khi 0,1 < X < 0,2: Co su xuSft hien cua tuong tac sat tir giiia cac ion Mn^^-Mn^"", tuy nhien sir pha tap nho nen tinh sat tiif chua dii manh va chua pha v6 dugc tinh chat dien moi

6 duai nhiet dp chuyen pha sat tit - thuan tii, trang thai trat tu dien tich dan dan dugc hinh thanh diin den sir xua't hien ciia ca'u tnic phan sat tir thay the cho cau triic sat tir

Khi 0,2 < X < 0,5: Tuong tac DE chiem uu the, hgp chat mang tinh sat tir kim loai Khi 0,5 < X < 0,9: Su d6ng t6n tai va canh tranh giiia tuang tac DE va SE trong hap

chat dugc the hien ro n6t Kfift qua Ik su t6n tai chuyen pha trat tu dien tich a nhiet dp thap

duai nhiet dp T^

12

true tiep vao vi tri Mn (B) d^u gay nen nhihig thay doi co ban ve tinh chat vat ly ciia tirng

hgp chat Nhiing thay d6i nky da c6 s6c 16i cuon cac nha khoa hpc ca trong linh vuc nghien ciiu co ban vk ihig dung V6 nghien ciiu co ban can phai tlm hieu ngu6n goc va co che cua nhirng thay d6i tinh cWft vat lieu Y6 mat nghien ciiu ling dung, dieu quan trpng

nhat la phai nang cao dugc nhiet d6 chuyen pha Curie (Tc) ciia cac hgp chat perovskite

manganite len viing nhiet dd phong vk nang cao ty so tii dien tro ciia chiing nhat la trong

nhirng viing tii truing th^p Mat khic, hieu ling tii nhiet Idn ling dung trong cac thiet bi lam lanh the' he mcfi ciing da dugc nghien ciiu manh va dat niem hy vpng vao vat lieu perovskite chiia mangan

Hgp cha't LaMnOg thi hien tmh phkn sat tii do tuong tac SE giua cac ion Mn^^ Khi pha tap kim loai ki6m th6 Ca (hoa tri 2) vko vi tri dat hiem, de can bang hoa tri thi mot

phan tuong duong ion Mn^"" chuyen thknh Mn"^"" Khi do trong hgp cha't xuat hien ca tuong tac SE giiia cac ion cimg hod tri (Mn"^- Mn"*^; Mn"^^- Mn""^) va tuong tac DE giira cac ion

khac hoa tri (Mn"*"^ Mn"*^) Nhtf vay ck hai loai tuong tac DE va SE ciing t6n tai va canh

tranh nhau trong hop chaft pha tap Lai.xCaxMn03 Ca'u triic tir va tinh din dien ciia vat lieu perovskite Lai.j^CaxMn03 dugc quy^t dinh bofi cu5ng dp va tuong quan giira hai loai tuong tac nay Cuofng dp va tuong quan giOa hai loai tuong tac nay lai phu thupc vao n6ng dp thay the Ca cho La trong hop cha't Vdi m6t qua trinh nghien ciiu ti mi va cong phu bang thuc nghiem Schiffer ciing cdc dong nghiep da xay dung dugc gian d6 pha ciia he La, ,Ca,Mn03 nhu tren hinh 2

Khi chua co su pha tap (x = 0) thi hgp cha't co tinh phan sat tir dien moi vai cau iriic phan sat tii* kieu A

Khi CO su pha tap vdfi x < 0,1 thi tuong tac giira cac ion d6ng hoa tri Mn^^- Mn''\ Mn^'^- Mn^"^ chie'm uu the', he mang tinh phan sat tii' voi cau triic CAF

Khi 0,1 < X < 0,2: Co su xuSit hien cua tuong tac sat tir giira cac ion Mn^'^-Mn'^'', tuy

nhien sir pha tap nho nen tinh sat tii chua du manh va chua pha vo dugc tinh chat dien moi

d duai nhiet dp chuyen pha sat ti^r - thuan tii, trang thai trat tu dien tich dan dan dugc hlnh thanh dan den su xua't hien cua cau tnic phan sat tir thay the cho cau tnic sat tir

Khi 0,2 < X < 0,5: Tuong tac DE chi^m uu the, hgp chat mang tinh sat tir kim loai

Khi 0,5 < X < 0,9: Su d6ng t6n tai vk canh tranh giira tuong tac DE va SE trong hgp chat dugc the hien ro n6t K^t qua la su t6n tai chuyen pha trat tu dien tich a nhiel dp thap

Khi 0,9 < X < 1: Tuong tdc SE lai tr& nen chiem uu the, vat lieu the hien tinh phan sat tii dien mdi vod c^u tnic CAF

Khi X = 1: Su pha tap la hoan toJin, hgp ch^t lai tro thanh phan sat tir dien moi voi cau tnic

phan sat tir kiin G

Hinh 2 Gian dd pha cua he La,,^CaJ\4nOj

3 Noi dung chinh

Bao gom hai phan: Nghien curu khoa hoc

Dao tao 3-1 Nghien ciiti khoa hoc

3.1.1 Nghien ciiu qui trinh che tao m l u Perovskite ABO3 cau tnic don pha

Chung t6i da tie'n hanh che tao he miu Lai.^Ca.MnO, (x = 0,10; 0,20; 0,30; 0,40; 0,46 va 0,50) bang phuong phap phan ling pha ran thong thuong

Phirong phap g6m truy^n thtfng dugc tie'n hanh theo cac quy tac chung da neu a

phan thuc nghiem Viec che'tao dugc ti6h hanh cu th^ nhu sau :

Tir cac ph6'i lieu ban d^u nhu mu6i MnC03, CaC03 va oxit La203 vai dp sach cao (3N ^ 4N) Cac ph6'i lieu dugc tinh todn va can theo hgp thirc danh dinh la Laj.,Ca,MnOj.,i

(voi X = 0,10 -^ 0,50) Quy trinh cong nghe che' tao mau dugc tien hanh theo cac buoc sau:

Budc 1: Nghien trpn cdc b6t 6xit nay tur 60 -^ 90 phiit, say kho h6n hgp bpt a 200 '*C trong khoang 9 -^ 10 gid sau d6 nghien lai trong khoang 30 phiit va ep thanh vien co duong kinh O co 15 -^ 20 mm va d6 dky cd 1,5 -^ 2 mm, mau dugc say kho ca 4 gia 6 100

13

"C ^ 200 "C, miu dugc nung so bg d nhiet d6 1100 ^C trong thoi gian 24 gio, sau do mau dugc de ngupi theo 16 d6n nhiet dp ph6ng

Budc 2: Sau khi miu da ngudi, l^y mSu ra khoi 16 va nghien lai mau trong khoang

1 gio, ep lai thanh vien, sau 66 sáy kh6 nhu of budc 1 Nung thieu ket mau 6 nhiet dp 1000

"C trong 2 ^ 5 gid va 6 nhiet đ 1300 °C trong 10 H- 20 giọ Cuoi ciing la cong doan ii mau

6 650 '*C trong th6i gian 48 gi6 roi d^ ngu6i dén nhiet dp ph6ng

Cac budc ti^n hknh chfif tao mSu dugc m6 ta ngan ngpn bang so d6 duai day:

Can phd'i lieu

^ f

Nung so bp

3.1.2 Nghien ciiu cac tinh chat chuyen cua vat lieu che tao

• Nghien ciiu cau triic: De nghien ciiu cau triic cua cac mau chiing toi da tien hanh do cac phep do: Chup anh SEM, pho tan sdc nang lugng (EDS) va nhi^u xa X - Raỵ

a) Phep do dp kich thirdc hat tinh the bang chop anh SEM:

= 0*10 ^ ^ B W M l f e l l P ^ i ^ = 0,20 i ' ^ 'm ' <^^ ^ ^ X = 0i30

Hinh 3 Anh hien vi dien tii quit (SEM) cua cck^ mau La/^^CâMnỘo

Tir anh SEM (hinh 3) cua cad mSu cno thay hat tinh the hlnh thanh la d6ng nhat Vai sir tang ciia ham lugng Ca thay th^, vdi x = 0,10; 0,20 va 0,30 hat tinh the Ian hem vai

kich thudc x^p xi l^ira tham chi c6 m6t s6 hat co kich thuoc Ion horn Mat khac, ta thay

voi ham lugng Ca tang len trtn be mat mSu hinh thanh cac hoc trong va bien hat khong ro

b) Phep do pho tan xa nang lirong (EDS) de xac dinh thanh phan va dp sach cua

Hinh 4 Gian dd nhieu xa ndng luang (EDS) cua cac mau

Laj.,Ca,MnOj,s(v^x = OJO vd 0,20)

Ph6 EDS (hlnh 4) ciia t^t ca cdc mSu deu cho thay khong co dinh la va mau chiifa

cac thanh phan La, Ca, Mn vk Oxy Khi h ^ lugng Ca thay the cho Lanthan (La) tang tii' x

= 0,1 den x = 0,2, cu6ng d6 cua cac dinh dac trung cho Ca cung tang dan

c) Phep do nhieu xa Rofnghen (X - ray) de xac dinh cac hang so mang va dp don

pha cua mau

Hinh 5 la gian d6 nhiSu xa Roughen cua he mau La,.,Ca^Mn03.5 (vai x = 0,10 -^

0,50) sii' dung bii'c xa Cu Ka v6i bu6c song A, = 1.54056 A Gian do nay chi ra rang cac

mau khong chiia cac pha la Cdc dinh tren cac mSu sac net chiing to cac mau che tao la

dan pha vdi ca'u tnic tinh the' thudc nhom perovskite dang true thoi (orthorhombic).Tu' gian d6 Roughen ta co thi tinh dugc cac hang so mang ciia miu La,.^Ca,Mn03.6 (v6i x =

0,10-f 0,50)

IS

20 30 40 50 60

26

Hinh 5 Gian do nhiiu xa Ronghen cua cac mau Laj.^CâMnOj.s vdi x = 0,10 ^ 0,50

• Phep do thanh phan khuyet thieu Oxy trong mau

Bang phuong phap Dicromat, thanh ph^n khuyet thieu oxy (5) ciia cac mau La,, CạMnỘa (vdfi x = 0,10 -^ 0,50) da dugc xac dinh Tií cac gia tri nay chiing ta ciing co the linh dugc lugng Mn*^"^ va Mn^"*^ tren m6t don vi cong thiic bang phuong phap can bang hoa iri trong m6t hgp ch^t, tiic la t6ng hoa tri cua cac ion am va ion duong trong mot hgp chat bao gio ciing bang khdng Tren nguyen tac do, ta co phuong trinh tinh so lugng ion Mn^^ va ion Mn"^"^ trong m6t don vi hgp thiic nhu sau:

So ion LáMrong Stf ion Câ trong Stf ion M n ^ Irong

inoi it(Tii \ i http ihiJc inO* tién vi hgpilifc mpt dcJn vj hop ihuc

So ion M i l " irong innl Jon vi lufp Ihuc

1 1 I

IIIx(l-x) + H x x + m x a + I V x ( l - a ) I I x ( 3 - 6 ) = 0^

f

Thay cdc gid tri cua x vac phuong trinh nay ta se tim dugc so ion Mn^^ (a) va so ion Mn'*^ (1 -a) CO trong mdt don vi hgp thtic, tii do suy ra ti s6 ion Mn^^/Mn^"^

Bang i Cac gid tri S, Mn^* vd Mn"** vd ti soMn^^/Mn'*^ cua cac mau

Mn^-^

0,9176 0,8402 0,7261 0,4490 0,5758 0,4612

Mn^^

0,0824 0,1598 0,2793 0,5510 0,4242 0,5586

Mn'^A4n'^

11,136 5,258 2,56 0.815 0,8692 0,8256 Chuyen pha sat tur thuan tur

Chiing t6i da do duong cong tii đ phu thupc nhiet dp voi hai che dp lam lanh co tir truang (Field Cooling-FC) va khdng c6 tit trudng (Zero Field Cooling-ZFC) a mot tir

truang thap (Ik Oe) cho tix ca cic mSụ Hinh 6 la cac duong cong FC va ZFC ciia cac mau

khao sat Tii" cac duofng cong tii nhiet nay chiing toi da xac dinh dugc nhiet dp chuyen pha sat til" Tc cho cac mSụ

'^iUSCtiJ^i^ji^

FC

x^ỌIO

1 ZFC

Hlnh 6: Tii đ phu thudc nhiet đ ciia cac miu Laj.CâMnỘ^ ( x = 0.1 va 0,2)

Mat khac, tit cac dudng cong FC va ZFCT ta thay sir tach nhau ro ret giira hai duang

FC va ZFC a viing nhiet dp th^p Di^u nay co thé dugc hieu la do co sir khac biet cua hai

17

£^Ai HOC QUOC GIA HA

NO-•PUNGTAMTH^NGTlNiTụr v,f ^

qua trinh lam lanh miu khi c5 va khdng co tft truofng Khi mau dugc lam lanh trong tir

truong (FC), cac spin h6n loan va linh ddng d trang thai thuan tir dugc dinh hudfng trong tir truong va khi lam lanh cdc spin nky hi ddng bang nhung van con nhirng dinh hudfng cu, nen gia tri tix dd cua dudng FC thudng Idfn Khi lam lanh khdng cd tir truong (ZFC), cac

spin hdn loan 6 trang thdi thuan tit khdng dugc dinh huong nen khi lam lanh sir hdn loan cua cac spin la gifl nguydn vi vay gia tri tii dd ciia cac dudng ZFC thudng nho hon so voi

FC Trong tii tnrdng nho, su tach nhau cua dudng cong FC va ZFC d mot nhiet dp nao do dugc gpi la nhiet dd thuan nghich T^ (< Tc) Mat khac, d tit trudng thap cac dudng cong tir nhiet ZFC deu co mdt cue dai d nhiet dd Tg (nhiet dd chuyen pha spin glass, Tg < T,) dugc gpi la nhiet dd dong bang spin Khi tii tnrdng tang thi nhiet dp T^ dich ve phia nhiet dp thap va cue dai trdn dudng ZFC chuyen thanh mdt cue dai rpng Cac hien tugng do xac dinh dac tinh ciia mdt trang thai thuy tinh spin hay thuy tinh dam

Hieu ling tur nhiet:

Su thay d6i entropy tit do trat tu spin anh hudng bdi tir trudng ngoai dugc tinh theo cong ihiic nhiet dong sau:

Hinh 7: Su bien thien entropy tie (ASJ cua cac mau Laj.^Ca^MnOj.s (vdi x = OJO

0,2) diidi tac dung cua tit trudng ngoai H - 1T,3T va 5T a) x = 0,10: b) x = 02

Hlnh 3.9 trinh bay su phu thudc cua bie'n thien entropy tir vao nhiet dp ciia cac mSu nghien ciiu ung vdi dd bie'n thien tit trudng la 1, 3 va 5T Tir hp cac dudng cong tir hoa dang nhiet, gia tri bien thien entropy ttt da dugc xac dinh cho cac mau vdi tir trudng bien ihien cue dai dat 5T Tii dirdng cong AiS^,^ phu thupc vao nhiet dp, chiing ta co the tinh dugc khoang nhiet dd sir dung cua cac mSu la rat rpng: 25 K; 50 K Ian lugt tuong irng vai

cac mau CO X = 0,10 -^ 0,20 diiw nky co y nghia rat quan trpng trong viec ling dung vat

qua trinh lam Izmh miu khi c6 va khdng co tii trudng Khi mau dugc lam lanh trong tir trudng (FC), cac spin hdn loan va linh ddng d trang thai thuan lit dugc dinh hudng trong tir trudng va khi lam lanh cdc spin nay bi ddng bang nhung van con nhirng dinh hudng cu, nen gia tri tit dd ciia dudng FC thudng Idn Khi lam lanh khdng co tit trudng (ZFC), cac spin hdn loan d trang thai thuan tCr khdng dugc dinh hudng nen khi lam lanh sir hdn loan ciia cac spin la giir nguydn vi vay gia tri tit dd cua cac dudng ZFC thudng nho hon so vdi

FC Trong tii trudng nho, su tach nhau ciia dudng cong FC va ZFC d mot nhiet dp nao do

dugc gpi la nhiet dd thuan nghich T^ (< Tc) Mat khac, d tit trudng thap cac dudng cong tir

nhiet ZFC deu co mdt cue dai d nhiet dd Tg (nhiet dd chuyen pha spin glass, Tg < T^) dugc gpi la nhiet dd ddng bang spin Khi tit trudng tang thi nhiet dp T^ dich ve phia nhiet dp

thap va cue dai trdn dudng ZFC chuyin thanh mdt cue dai rpng Cac hien tugng do xac

djnh dac tinh ciia mdt trang thai thuy tinh spin hay thuy tinh dam

•> Hieu ling tur nhiet:

Su thay d6i entropy ttr do trat tu spin anh hudng bdi tir trudng ngoai dugc tinh theo cong thiic nhiet ddng sau:

thien cue dai dat 5T Tit dirdng cong ^max phu thupc vao nhiet dp, chiing ta co the tinh

dugc khoang nhiet dd sir dung ciia cac mSu la rat rpng: 25 K: 50 K Ian lugi tuang ling vdi

cac mau CO X = 0,10 -^ 0,20 di^u nky cd y nghia rat quan trpng trong viec li'ng dung vat

18

lieu nay dimg cho cac may lim lanh tii vdi nhilu uu viet hon ban so vdi cac may lam lanh sir dung chu trinh nen khi thdng dung That vay, vat lieu lanh tii tich cue ngoai viec cd bien thien entropy tit cao (cd su thay ddi nhiet dp Idn khi thay doi tir trudng) con rat can

cd khoang nhiet dd six dung rdng di cd thi ling dung trong cac may lam lanh

Viec che' tao dugc cac mau cd gia tri AiS^^ Idn cd y nghia rat quan trpng trong

viec lam lanh tit, no se nang cao hidu suSft ciia cac may lam lanh tix Nhu vay ngoai viec

che tao dugc cdc miu cd cd gia tri A^^^^ Idn chiing toi cdn ndi rpng dugc khoang nhiet

dp sir dung cua cac mSu, vide nay se lam cho kha nang sir dung ciia cac hgp chat nay vao viec lam lanh tii mdt each rdng rai hon Ttr cac gia tri AS^,,, ma chiing toi thu dugc cho thay cac mSu da chd' tao la thich hgp cho vat lieu lam lanh tir tich cue cd viing nhiet dp boat ddng gan nhiet dd phong

• Hieu ling trat tu dien tich

Trat tu dien tich (charge ordering - CO) la hien tugng quan sat dugc trong cac hgp chat ma d dd cac electron trd nen dinh xii do cac ion co cac dien tich khac nhau sap xep

mpi each co trat tir tren cac niit mang, tvt dd vat heu mang tinh chat dien moi CO xay ra d

mot nhiet dp tha'p hon mdt nhidt dd dac trung gpi la nhiet dp trat tu dien tich TCQ DO sir dinh xii" ciia cac hat tai dien nen tai Tco ta thudng quan sat thay dien trd ciia vat lieu tang dot ngpt, tire la d dd xua't hidn hieu ling tit

trd khong 16 (hinh 8)

Ddi vdi vat lieu La,.;(A\Mn03,

hieu irng CO the hidn nhu the nao la tuy

thudc vao su thay doi ciia ban kinh ion

trung binh d vi tri A (<rA>) va ti so

Mn^VMn-^ Ndi each khac, hien tugng

CO phu thupc vao loai ion pha tap va

nong do pha tap CO thudng quan sat th^y

d ndng dp pha tap x = 0,5 ciia vat lidu

Ln,sA^,sMn03 khi ti sd' Mn'^A^n^^ = 1

Tuy nhien hieu ung CO cdn dirge quan

scit trong mot sd perovskite khi ndng dd

pha tap 0,3 < x < 0,75

100 200 300

T ( K )

Hinh 8: Sit phu thuoc ciui dien trd sua) ciia

Pr(),,Cat,jMnO^ vao nhiet do tai car ti( tridm^

• Hieu umg tiir diSn trof trong perovskite manganite

Tix trd (magnetoresistance - MR) la mdt dai lugng cd lien quan den su thay doi cua

dien trd su^t hay đ dSn didn cua vat lieu khi chiu tac dung ciia tir trudng ngoai, va dugc xac dinh bang ti sd:

trong đ p(H) vk ff(0) Idn lugt la dien trd su^t ciia vat heu khi cd tir trudng va khi khdng cd

tir trudng Thdng thudng ngudi ta do dien trd ciia miu khi cd va khdng cd tir trudng ma khong do dien trd suát vi cd kha nang xay ra hieu litig tir giao lam bien doi hlnh dang ciia mau dudi su tac dung ciia tit trudng Tii trd ciia cac vat lieu tir cd the do dugc bang phuong phap ph6 bien nhát la phuong phap bdn

mui đ Gia tri MR cd th^ duong hay

am Cac kim loai sach khdng tit tinh va

cac hgp kim thi hien hieu ling ttr trd

duong va phu thudc bac hai vao tix

trudng (H) Ngugc lai, MR cd th^ am

khi dat mot vat lidu ttr vao trong tii

trudng bdi vi su sap x^p b^t trat tu ciia

cac spin khi đ se bi pha vd va cac spin

trd nen dong nhát

Hieu ling tir trd khdng Id (giant magnetoresistance - GMR) la ket qua ciia qua trinh giam manh dien trd do su tan xa electron khi cac spin sap xep khong phai la sat tir trong tir trudng khong (zero field) Cdc nghien ciiu cho thay vat lieu perovskite loai manganite the hien rat r5 hieu ling tií trd khdng Id Mdt dac diem kha quan trpng ciia hieu ling tir trd đi vdi ciic vat lieu manganite la nhiet đ ling vdi cue dai dudng cong dien trd phu thupc nhiet

dp (T^ax) hau nhu triing vdi nhiet đ chuyen pha Tc- Nhiet dp T^^, chinh la nhiet dp ma

tinh dan dien ciia vat lidu chuyen tit din kim loai {dR/dT>0) sang tinh dan ban dan (dR/dT <0)vh thudng dugc gpi la nhiet đ chuyén pha kim loai - dien moị

Ve CO ban, hieu ling tit trd cd th^ dugc giai thich theo co che tuong tac trao doi kep

DẸ Chinh vi vay ma ban kinh ion trung binh tai vi tri A va nong dp pha tap cd vai trd

20

Hinh 9: Sit phu thupc MR(H) ciia mdu LaffjPbfy^MnỘ

• Hieu umg tur difn trd trong perovskite manganite

Tii trd (magnetoresistance - MR) la mdt dai lugng cd lien quan den sir thay ddi cua

dien trd su^t hay dd din dien cua vat heu khi chiu tac dung cua tir trudng ngoai, va dugc

trong dd p(H) vk p(0) lin lugt la di6n trd su^t ciia vat lieu khi cd tir trudng va khi khdng cd

tir trudng Thdng thudng ngudi ta do didn trd ciia miu khi cd va khdng cd tir trudng ma

khdng do dien trd sua't vi cd kha nang xay ra hieu ling tir giao lam bien doi hinh dang ciia

miu dudi su tac dung cua tit trudng Tii trd ciia cac vat lieu tir cd the do dugc bang phuong phap phd bien nhat la phuong phap bdn

mui dd Gia tri MR cd th^ duong hay

am Cac kim loai sach khdng tit tinh va

cac hgp kim thi hidn hidu ling tit trd

duong va phu thudc bac hai vio tit

trudng (H) Ngugc lai, MR cd ihi am

khi dat mot vat lidu tii vao trong tit

trudng bdi vi su sip x^p b^t trat tu ciia

cac spin khi dd se bi pha vd va cac spin

trd nen dong nha't

Hieu ling tir trd khdng Id (giant magnetoresistance - GMR) la ket qua eiia qua trinh giam manh dien trd do su tan xa electron khi cac spin sip xep khong phai la sit tir trong tir

trudng khdng (zero field) C&c nghien ciiu cho thay vat lieu perovskite loai manganite the

hien rat ro hieu ling tii* trd khdng Id Mdt dac diem kha quan trpng cua hieu ling tir trd ddi vdi cac vat lieu manganite la nhiet dd ling vdi cue dai dudng cong dien trd phu thupc nhiet

dp (Tniax) hau nhu triing vdi nhiet dd chuydn pha Tc- Nhiet dp T^.^^ chinh la nhiet dp ma

tinh din dien ciia vat lidu chuyen ttt din kim loai (dR/dT>0) sang tinh din ban din (dR/dT <0)vk thudng dugc gpi la nhiet dd chuyen pha kim loai - dien moi

Ve CO ban, hidu ling tit trd cd thd dugc giai thich theo co che tuong tac trao doi kep

DE Chinh vl vay ma ban kinh ion trung binh tai vi tri A va nong dp pha tap co vai tro

Hinh 9: Sir phu thudc MR(H) ciia mdu La^^yPb^^^MnO^

quyet dinh d^n đ Idn ciia MR Theo co ch^ DE, trong pha thuan tir, cac dien tir din se bi tan xa manh bdi su bat trat tu cia cdc spin djnh xii do vay vat lieu cd dien trd Idn va mang

linh dien mdị Trong pha sit tix trat tu, su sip xep song song cua cac spin ion dinh xii se

giup cac hat tai chuyen đng gin nhu tu do qua cac niit mang tinh the va do vay dp din cua vat lieu mang tinh kim loaị Khi cd mat ttr trudng ngoai, cac spin sip xep sit ttr mot each trat tu hon niia va dien trd vat heu se giam (ttr trd am) Gian do trdn hlnh 1.14 cho ta thay hieu ling tit trd am đi vdi miu perovskite LaôPb^ ^MnOv Co che nay cdn tien doan mot dinh cue dai ciia dien trd tai Tc- Tuy nhien cac sd lieu thuc nghiem cung nhu cac tinh loan ly thuyet lai cho ring md hinh DE li khdng dii de giai thich cac tinh chat din cua cac manganitẹ Cac sd lieu thuc nghiem cho thiy trong pha thuan tir vin cdn ton tai hieu ling tir trd am De giai thich co che nay, hieu ling lien két manh dien tir - mang lien quan den

cac meo mang JT da dugc dua vao di giai thich tinh chat din cua vat lieu perovskitẹ

3.1.3 Ket qua cu the

Sd cong trinh Khoa hoc da cdng bd: 7

+ Sd cdng trinh Khoa hoc cdng bd tren cac tap chi Quoc te: 01

-I- Sd cdng trinh Khoa hoc cdng bd trdn cac tap chi Quoc gia: 02

+ Sd cdng trinh Khoa hoc cdng bd trdn tuyen tap HNKH Quoc te: 02

+ Sd cdng trinh Khoa hoc cdng bd trdn txxyin tap HNKH Quoc gia: 02

[ 1 ] Nguydn Huy Sinh, Vu Thanh Mai, Dd Viet Thing

Nam:2006

Ten bai bao : Tinh chdft vat ly cua hgp chat thieu Lantan Laô^sCâj^MnỘs

Ten tap chi/TuyIn tap Hdi nghi, Sd, trang: Tap chi khoa hpc va cdng nghẹ

Tap 44, Sd 3; (2006) 123 - 130

Tom tat cong trinh bang tieng Viet:

Hgp chat Lantan Lao,45Că, 43Mn03.5 dugc che tao bang phuong phap phan ling pha ran Cac tinh chát vat ly cua vat lidu da dugc nghien ciiu trong viing nhiet dp tir 77K -350K - nhiet dp chuyen pha nhan dugc la 350K tií ca hai phep do ve sir phu thupc nhiet dp cua dp tir hoa va mdmen tu" ciui miụ Gia tri entropy thay đi cue dai dat dugc la 2,45J/kgK tai tir trudng 5T Dd thidu hut oxi xac dinh dugc la 5=0,0237 va ti sd ion Mn^VMn'^'' ciia miu la 0,3466 Dac biet la trang thai trat tu dien tich xuat hien tai Tco =

175 K va chuyen pha kim loai - dien mdi tai Tp=99 K Trong viing nhiet do 9 - 99K vai kieu bieu hien tinh kim loaị Cic tinh chit vat ly cua hgp chat nay dugc giai thich bang su canh tranh tuong tac giira tuong tac sidu trao đi (SE) va tuong tac trao đi kep (DE) khi thay đi nong dp Id trong trong hgp chit

21

[2] Nguyin Huy Sinh, Vu VSn KMi, Nguyin Thi Thuang

Ten bai bao: Tit tr& trong cdc hfp chat La2/3Cai/3Mn,.JM:,03 (TM = Cu va Zn)

Tuyln tap HOi nghj V|t ly chit rSn toan qu6c la thu 5, Vung Tau 12 - 14/11 /2007

Tom tat cong trinh bSng tieng vi?t:

Cac mSu don pha perovskite LaT/aCai/sMnj.xTMxOa (TM = Cu Zn) dugc che tao bang phuong phip phin ling pha ran d nhiet dp 1150°C Nhiet dp chuyen pha Curie (Tc)

dd dugc xac dinh va cho thay Tc giim theo sir gia tang ciia ndng dp pha tap Cu va Zn Cue dai tren dudng cong di$n trd cua cic miu tai Tp cho biet chuyen pha Kim loai - Dien mdi/ban dan Nhiet dp chuyen pha Tp thdng thudng thap ban nhiet dp chuyen pha Tc Tir trd Idn ciia cac mau xuit hi§n xung quanh Tp va dat dugc gia tri 23,4% ddi vdi mau pha tap Cu va 12,2% ddi vdi mSu pha t^p Zn vdi ndng ddng pha tap x = 0.20 trong tir trudng thap H = 0,0 - 0,4T Dudng cong tir trd ph\i thupc tir trudng d nhirng nhiet dp xac djnh cd dang chu V va tang len d vimg nhi^t dp thap Giai thich cho hieu ung tir trd Idn dugc lam sang to bdi cac nguyen nhan: sy suy giam tuong tac DE, dp linh ddng cua cac dien tii Cg

va ling suat ndi tai din den meo mang Jahn - Teller trong cac hgp chat

[3] Vu Van Khii,Nguyen Thj Thuang, Nguyin Huy Sinh, Dd Viet Thang

Ten bai bao: Hi^u irng thay the Coban nong do thap trong cac hap chat

La^^^Pb^i^MnOy

Tuyen tap Hdi nghj Vat ly chSt ran toan qude la thu 5, Vung Tau 12 - 14/11/2007

Tom tat cong trinh bang tieng vi?t:

Thay the 10% Co cho Mn trong hgp chat La2/3Pbi/3Mn03 va nghien ciru cac tinh chit ciia chiing Cac ket qua cho thiy rang miu co cau true hexagonal va thira sd dung han thay ddi khdng dang kl Nhiet dp chuyen pha Curie (Tc) xac djnh dugc la 345K giam lOK so vdi miu Ididng pha tap Nhiet dp chuyen pha kim loai - dien mdi/ban dan (Tp) giam rSt manh bdi Co thay the vj tri Mn Tir trd Idn trong cac mau nam trong viing nhiet

dp thap han Tp DiSu dac bi^t la da xac dinh dugc diem udn tren dudng cong tir trd phu thupc nhiet dp trung vdi nhi?t dp chuyen pha Tp Cac gia tri tir trd trong vimg tir trudng thap (H = 0,0 - 0,4T) d nhOng nliiet dp khac nhau ciing dugc xac dinh

• Van spin

[4] Hoang Du:c Quang, Nguyen Huy Sinh, SUhk - Kun OH, Huynh Thanh Nhan

Nguyen The Hidn, Jerry Zidanic va Seong - Cho Yu

Nam : 2006

Ten bai bao : Cache phan vtng cua cdc nguyen to (Co, Fe, Mn) ton tai trong cdc

van spin chita cdc lop oxit

Ten tap chi/Sach/Tuyfo tap Hdi nghi, Sd, trang: Tap chi vat iy ling dung Nhat Ban Tap 45, Sd 1 A; (2006) 88 - 92

Tom tat cdng trinh bSng tieng Viit:

Tren CO sd dac tinh hoi hpc cua cac nguyen to (Mn, Fe, Co, etc ) ton tai trong cac Idp van spin chiia cic Idp oxit, chiing tdi nghidn ciiu hieu irng cac co che phan ling cua cac nguyen to nay dua tren dac tmh nhiet ciia cac van spin Sir giam nhiet ciia van spin MMn -

CO sd (M = Fe, Co, Ir, R, etc ) d nhiet dd cao dugc gay ra bdi sir khuech tan cua thanh phin Mn trong ciu tnic Vai trd cua oxit Mn nhu hang rao khuech tan ngan can sir di chuyen cua Mn xac dinh dugc bing phuong phap phan tich dp nghidng quang phd ion thii cap (SIMS) va phd quang didn tir tia X (XPS) Su khuech tan Mn trong Idp tu do bi han che

d nhiet dp 300"C trong 1 van spin cd Idp oxit ma d dd xay ra sir tao thanh MnO trong Idp

oxit dugc xac dinh bing cac dinh l^^ vk 2pi/2 Khi Mn khuech tan tir FeMn (khdng cd Idp oxil)thfi khdng cd su tao thknh oxit nio dang ki Cac co che phan ling lien quan den cac

nguyen to hoa hpc tdn tai trong cac Idp oxit cd.the bie't dugc bing sir thay ddi nang lugng

tu do thoe quy tic Gibbs nhU 1 ham phu thudc nhiet dp ddi vdi cac nguyen to (Co, Fe, Mn, etc )

[5] Hoang Diic Quang, Cao Xuan Hun, Nguyen Huy Sinh, Suhk - Kun OH, Nguyen

The Hien, Vu Van Khai va Seong - Cho Yu

Nam : 2006

Ten bai bao: Thi hieu dung trao doi vd cdc hieu u:ng lien quan trong van spin

Ten tap chi/Sach/Tuy^n tap Hdi nghi, Sd, trang: Hdi nghi qude te Ian thir nhat ve Vat lieu va hdi nghj qude te lin thii 3 ve vat ly nano va cdng nghe nano Ha Long, 6

- 9 thing 12 nam 2006

Torn tat cong trinh bang tieng Viet:

Mot trogn nhiing hieu litig anh hudng de'n dac tinh dich chuyen tii* ciia cau tnic van spin la hieu ung dich chuyen qua lai ciia Meiklejohn va Bean xay ra d cac Idp ludng sit tir (FM) va phan sit tu: (AF) Mac dii da cd nhirng nghien cuu nhung vai trd ciia su bat binh dang hudng d be mat phan cich cua cac Idp sit tir va phan sit tir vin chua dugc lam sang

id De hieu ro su bit dang hudng d bd mat phan each, chiing tdi che tao ca cau tnic van spin va he dich chuyin qua lai 3 Idp Cac tinh chat tir va cau tnic vi md ciia van spin 3 Idp

da dugc khao sat bing phuong phap nhiSu xa tia X, dp nghieng qunag pho ion thii' cap (SIMS) va tir kd' miu rung (VSM)

•> Hieu ling xuyen ngam

[6] Hoang Diic Quang, Nguyen Huy Sinh, Suhk - Kun OH, P.T San, N Q Hoa V V

Khai S C Yu

Nam : 2007

Ten bai bao: Tinh dan cua bdn ddn xuyen ngam tit ca sa FeCo

23

Ten tap chi/Sich/Tuy^n tap Hdi nghi, Sd, trang: Hdi nghi Bic Kinh - Trung Quoc 2007

Tom tat cdng trinh bing tieng Viet:

Cic transistor hidu irng xuyen ngim ttr (MTTs) cau tao bdi Si(100)/CoFe-x/AlO,

4/CoFe-6/FeMn-20/Cu-5/ra5 (don vi nm) cd cac dp rdng co sd khac nhau dugc che tao va khao sat d nhiet dd phong Nhan thiy su bit thudng cua dp din hieu ling xuyen ngim tir gia tri dIc/dV trong dang tir hoi song song khi thay ddi the hieu V^.^ trong khoang 1,8 -2.1 V Sir bat thudng cua dd din phu thudc vao dang tir hoa dugc lam sang to nhd dieu chinh dp din hidu ling xuyen him bang su phan cue spin va su bat ddi xiing quang dudng

^ I •» A I

tu do trung binh ,X/X O day, xic dinh vai trd ciia X /X bing phuong phap phd quang

dien tir tia X (XPS) hoac he sd gdc ciia dd din hieu ling xuyen him tir cho ca dang tir hoa song song va ddi song Ngoii ra, thd hidu dich qua lai, HgR, va cac hieu irng kien quan cung dugc khao sat nhd dudng cong ttr trd hieu ling xuyen him khi thay ddi nhiet dp 100 -300° C

3.2 Dao tao

+ Hudng d i n thuc tap NCKH: 04 sinh vien (Dd Viet Thing, Nguyen Thi Nhung, Tran Thi Ha, Trin Qui Long) K49, sinh vien Khoa Vat ly Trudng Dai hpc Khoa hpc Tu nhien

+ Hudng d i n nghien ciiu Khoa luan tdt nghiep: 04 Cu nhan Dai hpc (Tran Thi

ha, Nguyen Thi Nhung, Trin Tuin Khdi, K49, Khoa Vat ly, Trudng DHKHTN Nguyen Thi Thuong, K50 Khoa Vat ly, Trudng DHKHTN

+ Huong d i n luan van Thac sT: 03 luan van Thac sT va 01 khoa luan Cao hpc

1 Nguyen Tuan Son Bao ve thang 11/2006

De tai: Anh hudng ciia su thay the' kim loai tir tinh 3d cho mn trong cac hgp chat Lao.,7Ca,),33Mn(,9oTMo.i03.6 (TM = Fe, Co, Ni)

Ccin bd hudng din luan van: PGS TS Nguyen Huy Smb

2 Vu Van Khai Bao ve thang 12/2007

De tai: Nghien ciiu tinh chit dien va tii" trong he hgp chat perovskite La2/3Caiy3Mn03.5 pha tap Zn

Can bd hudng din luan van: PGS TS Nguydn Huy Sinh

3 Tran Minh Diic

De tai: Mdt sd tinh chit chuyen trong he hgp chat Perovskite Lai.^Pb^MnO;,

Hpc vien da hoan thanh luan van nhung xin hoan de di cdng tac d nude ngoai

4 Pham Thi Thuong

Ten khda luan: Spintronics (2006)

+ Huong dSn lu^n &n Ti^n sT: 01

Vu Thanh Mai Bio v6 luSn an Tig'n si cSfp nha nu6c thang 5/2007

De t^i: Nghiin ciiu cdc chuyin pha vd hieu irng thay the trong cdc perovskite

manganite

Ma so: 62 44 07 01

Can b6 hu6ng din chuydn ng^nh: PGS TS Nguyen Huy Sinh

4 Ket lu$n

1) De tai da dat dugfc muc tieu d6 ra

2) Boi duang cho cdn bd cua B6 m6n nhung kien thiic ca ban ve chuyen nhanh vat Ueu tit

3) Nhung ket qua dugc tap hgp va chgn loc de lam tai lieu cho viec giang day chuyen

nganh b B6 m6n

4) Tap hgp tu lidu d^ chu^n bi vifit sdch chuyen nganh

5 Tai lieu tham khao

1) C N R Rao; B BaNeau; Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides Published by World Scientific Publishing

Co Pte Ltd (1998); ISBN 981 - 02 - 3276 - 4

2) Chahara, T Ohno, M Kasai and Y Kozono Appl Phys Lett 63 (1993) 1990

3) P Schiffer, A P Rcunirez, W Bao and S W Cheong Phys Rev Lett 75 (1995)

3336

4) C Zones Phys Rev 82 (1951) 403

5) H Y Hwang, S W Cheong, P G Radaelli, M Marezio and B Batlogg Phys Rev Lett 75(1995)914

6) W E Pickett and D J Singh Phys Rev B53 (1996) 1146

7) C H Chen and S W Cheong Phys Rev Lett 76 (1996) 4042

8) S H Lee and S W Cheong Phys Rev Lett 79 (1997) 2514

25

v.x/T^^^I CONG HOA X A H 6 I CHU NGHiA VIET NAM

D6c lap - Tu do - Hanh phuc

0 ^ TAI TRONG DIEM/DAC BI^T NAM 2006

• • •

1 Ten de tai.- Chi'tao va nghien cvtu cdc tinh chat chuyen trong vat lieu perovskite dan va kep co nhiet do Curie cao

Ma so; QG.06.05

Chii nhiem de tai: POSTS Nguyin Huy Sinh

Ca quan chij tri: Trudng Dai hgc Khoa lioc Tu nhien

Thai gian thuc hien: 24 thdng (5 12006 - 5 12008}

2 Ngay kiem tra tien d6: 0210212007

Dia diem: Khoa Vat ly, Trudng Dai hoc Khoa hoc Tu nhien

3 Thanh vien doan ki6m tra:

VephiaDHQGHN:

- TSKH Nguyen Dinh Diic, Truong Ban Khoa hoc - Cong nghe

- ThS Le Yen Dung, chuyen vien Ban Khoa hoc - Cong nghe

Ve phi'a Tnrong Dai hoc Khoa hoc Tu nhien:

- PGS.TS Pham Quoc Trieu, Truang Phong KHCN

- TS Tran Tlii H6ng, P Truang Phong KHCN

- PSSTS hfcjvu^^X T^' ^ ' ^ ^ F CAM hAco^ Khx>Q V^f / ^

4 Doan kiem tra da nghe chij tri de tai trinh bay tom tit ve tie'n do tricn khai thuc hien, noi dimg ket qua dat dugc tijr thang 5/2006 den thang 12/2006, tinh hinh kinh phi, kJio khan \ a thuan Igi trong thtJc hien

^ Si~{cc( ctcCfc ^>^ l^ho <^^ 'UjT/i < ^ ^'cfUr^ t^ Cona hS' 3 Jx^T 4oco

To cdi/c -^ -UrrunsL

•j)r-t€L< etc/'Grpo pAri^''^^^'^'^''^'^^^ ^Acr^ y;^-^^ j ^ ,

5 Danh gia va ket luan cOa doan ki6m tra:

Ha Noi, ngay OZ thang 02 nam 2007

Dai hoc

Qu6c gia Ha Noi

Truang Dai hoc Khoa hgc Tu nhien

/ P{^

Chu nhiem de tai

TSKH Nguyen Dinh Due TS Tf-c^n Tk,' ^'^ f^^f^- /^ft^^o ^^ '^iM

ISSN 0866 708X

VIEN KHOA HOC VA CONG NGHE VIET NAM

VIETNAMESE ACADEMY OF SCIENCE AND lECHNOLOfJ^

TAP CHI

KHOA HOC VA CONG NGHE

Tong bien tap:

Pho long bien tap

PGS - TSKH NGUYEN XUAN NGUYEN PGS - TS NGUYEN DUNG

PGS - TS PHAM HUU LY PGS - TSKH NGUYEN NGOC SAN

HOI DONG BIEN TAP

LE THANH BINH VIJ NGpC CUCJNG NGUYEN DUNG NGUYEN HUU DUNG NGUYEN CONG DINH NGUYEN VAN GIA TRUONG NAM HAI NGUYEN CONG HAO THAIHOANG

NGUYEN VAN HIEU LUU CAM LOC PHAM HUU LY

TS GS-TSKH PGS - TS

GS - TSKH GS-TSKH PGS-TSKH

GS - TSKH PGS - TS PGS-TSKH PGS - TS PGS - TS

GS - TSKH GS-TS PGS - TS

VU NHULAN TRAN DINH LONG CHAU VAN MINH NGUYEN CAO MENH

NGUYEN XUAN NGUYEN

LE VAN NHUONG NGUYEN HUU PHUONG NGUYEN NGOC SAN TRAN THANH SON PHAN MINH TAN

HO SI THOANG PHAM VAN THIEM PHAN THI TUOI

Bien tap ki thudt LE THI THIEN HUONG

a mure nhi^t dp phong va hieu ung tu nhiet Ion [1] Cac tai lieu cong bo da cho thay rSng mot

trong cac he hpp chat co kha nang sur dung vao vifc lam lanh tu do la LaịxCâMnOa [2] Ngoai vi^c dap urng dupe hai yeu cau tren, he vat lieu nay con co nhieu tinh ch5t hap dan ve mat nghien cuu ca ban va ung dung khac nhu: Co tu tra Ion, co chuyen pha trat tu tu, chuyen pha kim loai - di^n moi, chuyen pha cau tnic, chuyen pha trat tu dien tich [3, 4] Dieu dac biet hap dan trong

hp hpp chat nay la hpp chat thieu lantan co nhiet dp chuyen pha a vung nhiet dp phong, co hieu

ung tu nhi^t tuang doi Idn va nhi^t dp chuyen pha trat tu dien tich Tco kha caọVi vay nghien cuu cac hpp chat thieu Lantan ngoai viec tim hieu cac tuong tac vi mo con nham muc dich su dyng cac hpp chat nay vao vi^c lam lanh tu trong cac thiet bj lam lanh the h | mai [5 - 7] Mot trong nhung hpp chat dac biet do dupe chung toi nghien cuu cac tinh chat vat Ii ciia no la Laọ45Cao,43Mn03-5

IỊ THirC NGHIEM

Tu cac thanh phan ban dau la bpt La203, CaC03 va MnO dem trpn, nghien nho va ep thanh vien vai hpp thuc danh djnh Lao,45Cao,43Mn03.5 MIU dupe che tao bang phuong phap phan ung

pha tin Cac vien miu duoc nung o 1000*^C trong 20 gio va de ngupi den nhiet dp phong Sau do

mau lai dupe nghien nho, ep thanh vien va nung thieu ket a 1100°C trong 24 gio, 1250°C trong

15 gid va u eSỐC trong 24 gid rdi de ngupi theo Id Mau dupe nghien cuu bang cac phep do Ronghen, pho tan xa nang lupng (EDS), nong dp Mn^"^ va Mn'*", kinh hien vi dien tu quel, momen tu, he so tu hoa, dien trd va tir trd

IIỊ KET QUA VA THAO LUAN

0 0

m ,,

Gian do nhilu xa Ronghen cua mau Lao.45Cao.43Mn03^ trong hinh 1 cho thay miu co cai true orthorhombic va la don pha

Gia trj cac hiing s6 mang xac djnh dugc la a = 5,467A, b = 7,753 A, c = 5,443 A va dp mec cau true — = 1,418 • Cau true nay dupe dong nhit hoa vdi cau true khong gian Pnma Di^u nay

Hinh 2 Ket qua phan tich EDS cua mau Lao.45Cao,43Mn03.6

Hinh 2 la gian do phan tich EDS cua mau Gian do nay cung khang dinh rang khong co nguyen to la ngoai cac thanh phan La, Ca, Mn va O Ket qua phan tich xac dinh dupe cac thanh phan thuc cd trong hgrp chat: 42,44%La; 31,79% Mn va 25,77%Ca.Ket qua nay so vdi ti so danh djnh theo hop thuc La/Ca thap hon khoang 15%

Ngoai ra nghien ciiu anh hien vi dien tu quet (SEM) chup tren be mat mau cho thay su phan bo cac hat tren be mat mau la dong tinh va kich thudc cua chung tuong doi dong deu nhau Sir phu thupc nhiet dp cua dp tir hoa trong cac trudng hpp lam lanh cd tu trudng (FC) va khong CO tir trudng (ZFC) duoc trinh bay trong hinh 3 vdi H = 0,1 T Nhan thay ca hai dudng cong FC va ZFC deu gan nhu trung nhau va chiing toi xac dinh dupe nhiet dp chuyen pha sat tii (FM) - thuan tu (PM) d Tc = 305 K Nhu vay d tu trudng thap H = 0,1 T da khdng gay anh hudng nhieu den trat tu cac moment tir Gia trj Tc thu dupe d viing nhiet dp phong la hop chat Ii tudng CO the su dung vao viec lam lanh tu neu hieu ung tu nhiet cua vat lieu dii Idn

Tu hinh 3 cdn cho bi^t, khi giam nhiet dp, tren dudng cong M(T) xuat hien su chuyen trang thai trat tu dien tich tai Tco= 175 K d cac nhiet dp thap hon 175 K, vat lieu da chuyen tu sat tu thanh phan sat tu Bang chung ve su chuyen trang thai trat tu dien tich con duoc khang djnh tren

do thi do h? so tu hoa XAC (hinh 4) va dien trd theo nhiet dp d hinh 5 Tu hinh 4 co the nhan thay

su suy giam manh ciia dudng cong X'AC va X"AC- Ca hai dudng cong deu cho thay diem chuyen

pha trat tu di^n tich la 175 K va chuyin pha s^t tir - thuan iis d 305 K trung vdi kit qua thu duoc

d phep do moment tu Tren duong cong dien trd (hinh 4), su suy giam dot ngpt tai 175 K chinh

la bang chung ve su chuyin pha trat tu dien tich ton tai trong hpp chat nay

Gian do nhieu xa Ronghen ciia miu Lao.45Cao.43Mn03.6 trong hinh 1 cho thIy miu co cai true orthorhombic va la don pha

Gia trj cac hang s6 mang xac djnh dupe la a = 5,467A, b = 7,753 A, c = 5,443 A va dp mec cau tnic — = 1,418 • Cau triic nay dupe dong nhat hoa vdi cIu true khdng gian Pnma Dieu nay

Hinh 2 Ket qua phan tich EDS cua mau Lao.45Cao.43Mn03.6

Hinh 2 la gian do phan tich EDS ciia mau Gian do nay cung khang djnh rang khong co nguyen to la ngoai cac thanh phan La, Ca, Mn va O Ket qua phan tich xac dinh dupe cac thanh phan thuc cd trong hgp chat: 42,44%La; 31,79% Mn va 25,77%Ca.Ket qua nay so vdi ti so danl djnh theo hgp thiic La/Ca thap hon khoang 15%

Ngoai ra nghien cuu anh hien vi dien tu quet (SEM) chup tren be mat mau cho thay su phan bo cac hat tren be mat mau la ddng tinh va kich thudc cua chiing tucmg ddi dong deu nhau Sir phu thupc nhiet dp cua dp tu hoa trong cac trudng hpp lam lanh co tu trudng (FC) va khong CO tu trudng (ZFC) dugc trinh bay trong hinh 3 vdi H = 0,1 T Nhan thay ca hai dudng cong FC va ZFC diu gan nhu triing nhau va chiing toi xac djnh dugc nhiet dp chuyen pha sat tir (FM) - thuan tir (PM) d Tc = 305 K Nhu vay d tir trudng thap H = 0,1 T da khdng gay anh hudng nhilu din trat tu cac moment tir Gia tri Tc thu dupe d vung nhiet dp phong la hgp chat Ii tudng CO the sir dung vao viec lam lanh tir neu hieu img tir nhiet cua vat lieu du Idn

Tir hinh 3 cdn cho bilt, khi giam nhiet dp, tren dudng cong M(T) xuat hien sir chuyen trang thai trat tu dien tich tai Tco = 175 K d cac nhiet dp thip hon 175 K, vat lieu da chuyin tir sat tir thanh phan sit tir BSng chirng vl sir chuyin trang thai trat tu dien tich con dupe khang dinh tren

dd thi do h? so tir hoa XAC (hinh 4) va dien trd theo nhiet dp d hinh 5 Tir hinh 4 co the nhan thay sir suy giam manh ciia dudng cong X'AC va X'AC- Ca hai dudng cong deu cho thay diem chuyen pha trat tu di^n tich la 175 K va chuyin pha sit tir - thuan tir d 305 K triing vdi ket qua thu dupe

d phep do moment tir Tren duong cong dien trd (hinh 4), su suy giam dot ngpt tai 175 K chinh

la bang chung ve sir chuyin pha trat tu dien tich tdn tai trong hgp chat nay

24

Hinh 4 Su phy thupc ciia he so tu hoa dong theo nhiet dp cua miu Lao.45Cao.43Mn03.6

Co che de tim hieu ve cac tinh chat trong he hpp chat Lai.xCaxMn03.6 lien quan den sir truyen dien tich do cac sai hong mang tinh phi hpp thirc gay nen nhu cac niit khuyet, cation, su thay ddi ve ndng dp oxy hoac su pha tap Id trong thay the vj tri Mn Cd the nhan thay rang cac

.3+ _.-

nguyen to cd hda trj thap hon Mn cung cd kha nang gay nen su tao thanh cac cation Mn Nhu vay sir thay the cac cation Ca^" cho La^"^ tang dan se dan den lam tang cac trang thai dinh xir hon hpp Mn^^-Mn'*'^ trong hpp chat NghTa la tuong tac trao ddi kep (DE) sat tir cung tang len va he qua la nhi^t dp chuyen pha Curie (Tc) thuan tir - sat tir phai tang len Do dd ta thu nhiet dp Tc =

305 K Day la gia trj Tc Ii tudng cho viec su dung vat lieu vao viec lam lanh tir d nhiet dp phong Nghien ciru vl h^ thieu lantan Lai.xMn03.6, nhdm tac gia [5] da cdng bd Tc = 314,5 K khi x = 0,30 Nhirng ket qua ve nhi^t dp Tc dat dugc xung quanh nhiet dp phong ve he thieu La cung dugc cdng bo trong cdng trinh [6, 7] Tong hpp cac ket qua tren cd the cho thay rang sir gia tang ndng dp thieu La cung co thi la mot kha nang lam tang nhiet dp chuyen pha Curie Nhirng phan tich tren day cd thi dugc xac nhan bing cac kit qua thu dupe nhu phep do dp thieu hut oxy

5 = 0,0237, ham lugng Mn'* = 0,2577, Mn'* = 0,7426 va ti sd Mn'*/Mn'* = 0,3466 trong hgc chat Lao.45Cao.43Mn03.5 Ket qua nay dem so sanh vdi hgp chit co sd khdng pha tap LaMn03.6 co

8 =^p,03; Mn^' > 95% va Mn'" < 5% cho thay ring, sir gia tang cac trang thai hon hgp Mn'^

Mn la nguyen nhan chinh gay nen cac thay doi tinh chat cua vat lieu trong hgp chat thilu La Hon nffa trong hgp chat nay tuong tac thong trj da chuyin tir sieu trao doi (SE) sang tuong tac trao ddi kep (DE)

Thdng thuong khi lam giam dp rpng vimg din cua mot electron, trang thai trat tu dien tich

se la on dinh, vi khi dd tuong tac Coulomb giua cac nguyen tir ddng vai trd quylt dinh Dac diem chung cua cac oxit Mn - perovskite cd pha tap lo trong la tinh thi chju sir chuyin pha sit tir

- thuan tir d nhiet dp Tc do tuorng tac DE Do la sir lien ket FM giira cac spin difn tir 3d (trong trang thai t2g) thong qua trao ddi dong luc cua dien tii luu dpng 3d (trong trang thai Cg) Trong viing nhiet dp T < Tc, do su suy giam tan xa tir cua cac hat tai tich dien trong pha trat tu spin FM nen tinh kim loai cua vat lieu giam xudng din nhift dp chuyin pha Tco = 175 K Tai day dien trd

CO budc nhay va tang dot ngpt theo su giam ciia nhiet dp (hinh 5) Cac hinh 3, 4 va 5 cho thay ring tai noi co chuyin pha trat tu dien tich dong thoi ciing xay ra su chuyin pha tir sit tir (FM) sang phan sat tir (AFM) Vi vay trong viing nay T < Tco tuong quan sit tir giam va co su chuyin doi tir Spin sat tir sang Spin phan sat tir, dan din sir suy giam gia tri momen tir va he so tir hoa tren dudng cong M(T) va Xac(T)

tiinn > 5ir pnu tnuoc cua dien tra tneo nniet ap cua mau Lao,45Cao,43MnU3.5

Trat tu dien tich la trang thai dac biet hap dan bdi vi mot mat tuong tac trao ddi kep lam tang tinh kim loai cua trang thai sat tir, mat khac trang thai nay sat nhap vdi tinh dien moi cua trang thai phan sat tir hay thuan tir Do dd xuat hien sir canh tranh giua tinh sat tir kim loai trong trang thai FM va hif u irng Jahn - Teller tap the trong trang thai trat tu dien tich No tao nen birc tranh chung: trong trang thai trat tu djen tich sir meo mang orthorhombic trd thanh ben virng va ddng vai trd quyet djnh tinh chat cua vat lieu Nghien ciru dudng cong dien trd trong viing nhiet

dp thap tir 9 K den 120 K, chiing tdi tim duoc mpt cue dai ciia difn trd tai Tp = 99 K Trong vimg T < Tp (99 K) dudng cong dien trd bieu hien hoan toan la trang thai kim loai Nhu vay co

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